As is well known, transmitting picture information about moving pictures over telephone wires without any adaption of the information requires broad band. Therefore, a number of methods have been developed for compressing the information without greatly deteriorating the image quality .
Predictive DPCM coding (differential pulse code mudulation) is such a method of compressing the information. In this method the knowledge that there are only small differences between two successive images is utilized. In the receiver a predictor predicts what the next image looks like by means of the preceding image. A difference is formed and only the prediction error has to be transmitted to the receiver, which contains a similar predictor for reconstructing the original image.
In transform coding the image is divided into blocks of e.g. 8.times.8 or 16.times.16 picture elements or PEL. The image content is mathematically transformed into so called transform coefficients which are then quantized. The coefficients for gray areas are zero, according to this method, and do not have to be transmitted over the telephone wires.
DPCM coding and transform coding can be combined into a third method, in this application called hybrid coding (strictly hybrid DPCM transform coding), thus utilizing prediction as well as transformation. Hence, a block is transformed from its PEL domain or the time plane to its transform domain or the frequency plane by means of e.g. a discrete cosine transform (DCT). Then the transform coefficients are quantized and the prediction takes place in the transform domain with a similar loop as in the DPCM coding. This order may also be reversed so that the transformation takes place within the prediction loop and the prediction in the PEL domain, which is previously known. The transform coefficients, which can now take on discrete values between e.g. -128 and +127, are then detected in the frequency plane, the information carrying coefficients being concentrated to the beginning of the block. In addition, the probability for the different values of the transform coefficients is concentrated around zero, i.e. the probability for the value zero is greater than the probability for the values .+-.1 which is greater than the probability for the values .+-.2, etc.
Before the coefficients are transmitted they are coded with variable length coding, the structure of a block having been utilized such that the value zero, which is most probable, gets a code having the least number of bits, and higher values get a greater number of bits according to their probabilities. The terminating zeros of the block are not coded, but the end of the block is defined by a special character EOB (end of block).
The three methods above are described more closely in Televerket's technical periodical, Tele, vol. 91, No. 4, 1985, pages 1-7, and the periodical Elteknik, No. 14, 1986, pages 48-52.
A method and a means for variable length coding is described in my co-pending patent application Ser. No. 156,683 filed Feb. 17, 1988 entitled "Method and means for variable length coding".
The methods above may be divided into two modes, with prediction for all frequencies (interframe), and without prediction (intraframe).
Coding without prediction is specially advantageous in situations having scene changes and a large amount of motion, since a preceding image contains very little or no information at all about the next image. This mode is called "intra" in this application.
Coding with prediction is especially efficient for slowly moving pictures and unchanged background areas. This mode is here called "inter".
According to prior art, these two transmitting modes have been utilized, thus, for prediction for all frequencies or no prediction at all. The drawback to "inter" is that the prediction error is considerable for high frequencies, especially when there is much movement or changes in the images. According to the present invention savings can be made by transmitting certain frequencies with prediction and the remaining frequencies without prediction, i.e. by using a "mixed" mode.